1. Field of the Invention
The present invention relates generally to frequency conversion of laser light from fiber lasers. More particularly, the present invention relates to a device for the frequency conversion and mixing of laser light using a thermally and piezoelectrically tuned light source and a ring resonator to provide dramatic improvement in the available frequency range, uniform output intensity, and high conversion efficiency.
2. Related Art
The phenomenon of frequency conversion in nonlinear crystals has been studied since the 1960s and has long been recognized as a candidate for generating visible laser light. P. A. Franken et al., Phys. Rev. Letters 128, 1761 (1962); D. A. Kleinman, J. Appl. Phys. 39,3597 (1968); and G. D. Boyd et al., J. Appl. Phys. 39 3597 (1968). Nonlinear crystals have the property of doubling the frequency of a portion of the input energy. Nonlinear crystals can also act to mix two input sources to produce an energy beam having a frequency that is either the sum or the difference of the input frequencies. Several authors have shown that the performance of nonlinear crystals is improved when the crystal is situated in a resonating geometry such that the light repeatedly passes through the crystal. There are certain resonant conditions within these resonators at which frequency conversion is a maximum.
Fiber lasers provide an excellent source of infrared energy for coupling in external conversion cavities. Fiber lasers can provide a simple source of high power, single mode infrared energy. U.S. Pat. No. 5,974,059 to Dawson et al. describes using a fiber laser as an energy source for a linear resonator for frequency doubling which produces a modulated output. Linear resonator configurations have the drawback of reflecting some of the energy back into the source, requiring efforts to prevent reflected energy from damaging the system.
Frequency stabilization of laser light systems provides desirable signal sources for a variety of applications. Commercial and industrial systems often require lasers having a narrow linewidth, i.e. narrow frequency spectrum, and long-term stability, i.e. reduced frequency drift. Numerous control techniques have been used to stabilize the frequency by providing cavity lock through matching the input frequency with the resonant frequency of the cavity.
Some techniques include providing a frequency reference, such as an atomic or molecular resonator. Such a system passes laser light through a cell containing a gas and collecting absorption information. An advantage of this type of scheme is the highly stable frequency reference cell. However there are significant drawbacks in reduced reliability and limited linewidth.
One frequency stabilization scheme is the Pound-Drever (PD) method. For more information see Drever et al., Appl. Phys. B 31, 97-105 (1983). This method uses a phase discriminant to control the cavity lock of the resonator. Most often the PD method is used to move a mirror within the external cavity using piezoelectric control to adjust the cavity length. Other applications of the PD method have adjusted the laser source frequency using piezoelectric control. Such control schemes provide good results at narrow linewidths, but often suffer from long-term instability of the laser frequency. Attempts at providing long-term stability and narrow linewidths have met with varying success.
Therefore, an apparatus and method for frequency conversion which provides uniform and stable frequency output at narrow linewidths over a broad range with long-term stability would be considered a significant advancement in the art.
It has been recognized that it would be advantageous to develop a method which efficiently converts infrared light to produce uniform high power visible laser light.
In one aspect of the present invention, a device is provided for the nonlinear combination of laser light comprising a tunable fiber laser light source which produces an energy beam along an optical path at a given frequency, wavelength, and intensity, and includes thermal and piezoelectric elements for providing adjustment to the frequency of the energy beam; a nonlinear resonator placed in the optical path of the energy beam; a nonlinear optical crystal disposed in the optical path of the energy beam within the nonlinear resonator; and a feedback system operatively connected to the nonlinear resonator and to the thermal and piezoelectric elements of the fiber laser light source. The combination is configured to adjust the frequency of the laser light source to match resonant frequency conditions within the nonlinear resonator.
In accordance with a more detailed aspect of the present invention, the device includes a nonlinear resonator having a ring configuration consisting of four mirrors. Each mirror of the nonlinear resonator is positioned at an angle which is non-normal. An additional feature of the nonlinear resonator involves the use of selectively or partially reflective mirrors which increase conversion efficiency.
In accordance with another more detailed aspect of the present invention, the device includes a feedback system which involves a Pound-Drever control scheme to adjust the frequency of the energy beam to match the resonant frequency of the external cavity. The feedback system includes a detector which is responsive to light emitted from the external cavity and is connected to a phase modulator placed in the optical path of the energy beam after the fiber laser light source, for modulating sidebands to the energy beam. The detector is also connected to a controller coupled to the thermal and piezoelectric elements of the laser light source for adjusting the frequency of the energy beam.
In accordance with yet another detailed aspect of the invention a polarization rotator is placed in the path of the energy beam after the tunable fiber laser light source and before the phase modulator. A power amplifier is connected to the optical path before the nonlinear resonator for amplifying the intensity of the energy beam. A second polarization rotator is also placed in the path of the energy beam after the power amplifier and before the nonlinear resonator.
Finally, in accordance with another aspect of the present invention, a second tunable laser light source is configured to emit an energy beam and disposed such that the first and second energy beams are resonantly coupled to the nonlinear resonator for frequency mixing.
Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention.